It is estimated that 192,370 women were diagnosed with breast cancer in 2009 and 40,170 women died as a result of incurable metastatic breast disease. There is clearly a need to develop new diagnostic and therapeutic methods targeting both primary and metastatic breast cancer. Accumulating evidence suggests that Eph receptor tyrosine kinases and their cell-surface bound ligands, the Ephrins, play key roles in cancer progression. In particular, up-regulation of EphB4 expression has been found in mouse mammary tumor models and in more than half of the human breast cancer specimens examined. EphB4 is also widely expressed in human breast cancer cell lines, and has been found to correlate with survival in breast cancer patients. Based on the extremely important function of EphB4, therapies focusing on EphB4 have become potentially important components of breast cancer treatment strategies. However, tumor sensitivity to EphB4 suppression may not be uniform for all breast cancers, including primary v. metastatic disease. There is an urgent need to better predict which patients and individual tumors are likely to respond to such novel interventions, as well as monitor the therapeutic response. In this proposal, we plan to develop EphB4 specific PET probes, to quantify EphB4 expression in breast cancer xenografts. We hypothesize here that the EphB4 expression level is a determinant factor for predicting a tumor's response to EphB4 suppression therapy. Thus, PET imaging with suitably radiolabeled EphB4 antibodies, EphB4 inhibitors, or EphB4 binding peptides, will be highly valuable in assessing EphB4 suppression non-invasively and repetitively. The success of this novel imaging approach could lead to novel diagnosis method for breast cancer, help us better predict which patients and individual tumors are likely to respond to novel interventions targeting EphB4, and make it possible to direct monitor the responses towards therapeutic interventions. At the conclusion of this project, we will have accumulated sufficient animal-based translational data for submission of a clinically-based proposal. In addition to breast cancer, EphB4 is also significantly over-expressed in melanoma, prostate cancer, colon cancer, bladder cancer, lung cancer, mesothelioma, uterine cancer, and osteosarcoma. These newly developed PET probes could also have important applications in these other cancer types, and thus have a significant clinical impact on a very large number of cancer patients. PUBLIC HEALTH RELEVANCE: Breast cancer is the most frequently diagnosed cancer in women. It is estimated that 192,370 women were diagnosed with breast cancer in 2009 and 40,170 women died as a result of incurable metastatic breast disease. There is clearly a need to develop new diagnostic and therapeutic methods targeting both primary and metastatic breast cancer. The proposed research will develop EphB4 specific PET probes, which would be able to examine the breast cancer onset and progression by quantifying EphB4 expression non-invasively and repetitively. The data obtained from this project will help in many aspects of anti-EphB4 breast cancer therapy, particularly for patient stratification (e.g., selecting EphB4-positive cancer patients for new anti-EphB4 clinical trials while sparing EphB4-negative patients for other treatments), treatment monitoring, and dose optimization. The same probes developed in this project may also have important applications in many other cancer types, and thus have a significant clinical impact on a very large number of cancer patients.